Gog and Magog
The Emperor Diocletian had thirty-three infamous daughters, who murdered their husbands; and, being set adrift in a ship, reached Albion, where they fell in with a number of demons. The offspring of this unnatural alliance was a race of giants, afterwards extirpated by Brute and his companions, refugees from Troy. Gog and Magog, the last two of the giant race, were brought in chains to London, then called Troy-novant, and, being chained to the palace of Brute, which stood on the site of our Guildhall, did duty as porters. We cannot pledge ourselves to the truth of old Caxton's narrative; but we are quite certain that Gog and Magog had their effigies at Guildhall in the reign of Henry V. The old giants were destroyed in the Great Fire, and the present ones, fourteen feet high, were carved in 1708 by Richard Saunders.
Children used to be told (as a very mild joke) that when these giants hear St. Paul's clock strike twelve, they descend from their pedestals and go into the Hall for dinner.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894